Polymers obtained using an aromatic dicarboxylic acid and an aliphatic diol compound as monomers, for example, polyesters typified by polyethylene terephthalate (PET) and others are excellent in transparency, mechanical properties, melt stability, flavor retention, recyclability, etc. Thus, currently, such polymers are widely used as various wrapping/packaging materials such as films, sheets, hollow containers, etc.
However, since polyesters are not always sufficient in point of gas-barrier performance against oxygen, carbon dioxide and the like, use of polyesters as packaging containers is limited.
There have been employed some methods for improving the gas-barrier performance of polyesters. One method includes coating a molded article and a package container made of polyester with aluminum oxide or silicon oxide through vapor deposition. Another method includes coating a molded article and a package container made of polyester with a resin having gas-barrier performance higher than that of polyester. However, such coating methods have problems, such as addition of a cumbersome step to production of target polyester articles, and impairment in recyclability and mechanical properties of polyester products. Thus, limitation is imposed on carrying out such methods.
One method for easily improving the gas-barrier performance of polyesters while solving the aforementioned problems includes melt-mixing of a polyester resin with a thermoplastic resin having high gas-barrier performance. An example of the resin having such high gas-barrier performance is an ethylene-vinyl alcohol copolymer resin. The ethylene-vinyl alcohol copolymer resin has poor compatibility with polyester, due to a characteristic intrinsic to the molecular structure thereof. When the two resins are mixed together, the formed resin composition assumes a turbid state, impairing transparency, which is a merit of polyester. When an ethylene-vinyl alcohol copolymer resin is processed at a temperature suitable for processing polyethylene terephthalate, which is one of the most generally used polyesters, the ethylene-vinyl alcohol copolymer resin rapidly deteriorates. In some case, gelation and scorching occur, and the formed undesired matter is incorporated into final products, possibly impairing appearance and yield of the products. In addition, in order to remove the undesired matter from a production machine, disassembly and cleaning of the machine must be frequently carried out. Thus, difficulty is encountered in carrying out, on an industrial scale, the technique using ethylene-vinyl alcohol copolymer resin.
Examples of other gas-barrier resins than ethylene-vinyl alcohol copolymers include polyamides, typically nylon 6 and nylon 66.
Among such polyamide resins, polymetaxylyleneadipamide (MXD6), which is formed through polymerization of a diamine component mainly including metaxylylenediamine and a dicarboxylic acid component mainly including adipic acid, is a polyamide resin especially excellent in gas-barrier performance. Polymetaxylyleneadipamide has a glass transition temperature, a melting point and crystallinity that are almost equivalent to those of polyethylene terephthalate, which is one of the most generally used polyesters, and in addition, the heat stability thereof in melting is excellent and the processability thereof is good.
From these points, polymetaxylyleneadipamide can be said to be a resin particularly suitable as a material for improving gas-barrier performance of polyesters.
However, a molded article produced using a resin composition containing a polyester and a polymetaxylyleneadipamide is insufficient in point of transparency, and its use in applications requiring high transparency is therefore limitative.
PTLs 1 and 2 disclose a resin composition containing a polyester and a polymetaxylyleneadipamide, and intend to provide such a resin composition capable of solving the above-mentioned problems and capable of being a molding material for a molded article having high transparency by using a modified polyester that is modified with cyclohexanedimethanol (CHDM) or spiroglycol (SPG) as the polyester.